The present invention relates, in general, to power sources for hearing aids and, more specifically, to capacitor power sources, metal-air batteries and switches for batteries.
Batteries may serve as a power source for hearing aids. In some hearing aids the electronics of the device are placed inside the battery chemistry. Because the battery cannot be replaced, the hearing aid must be discarded when the battery energy is depleted. A target life for such a disposable hearing aid may be 30 days. Most hearing aids use replaceable batteries. The replaceable battery may be inserted into the hearing aid, thereby providing the power source to operate the device.
Replaceable zinc-air batteries are commonly used to power hearing aids. Prior to use, the battery is sealed with a pull-tab that prevents environmental effects, such as relative humidity and temperature, from affecting the shelf-life of the battery. To activate the battery, the pull-tab is removed and air (hence oxygen) is allowed into the battery. The battery is then inserted into the hearing aid to provide the power source for operating the hearing aid.
The nominal open-circuit voltage of a zinc-air cell (battery) may be 1.4 volts. The open-circuit voltage is measured after the pull-tab is removed and oxygen is allowed into the cell. Prior to removing the pull-tab, the open-circuit voltage of the zinc-air cell may also be 1.4 volts, because there may be some oxygen trapped inside the cell. As long as no current is allowed to flow through the cell, the cell voltage will remain at 1.4 volts and the shelf-life period will not be shortened. In disposable hearing aids in which the load (circuit) may be permanently connected to the battery, current may flow through the battery. The battery will then discharge and the desired shelf-life period may not be achieved.
It has been proposed that if oxygen was completely depleted from the cell, a long shelf-life would be achieved. This hypothesis is based on the assumption that there would be no potential difference to cause current to flow through the load in the absence of oxygen. It may be shown, however, that the voltage of some oxygen deprived zinc-air cells is not zero volts, but approximately 0.39 volts. As a potential difference may be present, a load connected to the cell will cause current to flow, thus discharging the cell.
Metal-air cells, such as zinc-air or aluminum-air cells, use air to activate the cell. A typical air cathode may be composed of four primary components:
(1) A carbon matrix formed by activated carbon blended with an aqueous Teflon slurry, washed, dried, and pressed into a current collector; the carbon matrix may include a catalyst, usually a transition metal oxide;
(2) a nickel mesh which provides mechanical strength and serves as the current collector;
(3) a microporous, hydrophobic membrane, typically polytetrafluoroethylene; and
(4) an anode/cathode separator which prevents direct contact between the anode and cathode.
Zinc-air cells are activated when air, and in particular oxygen, is allowed to enter the cell. In some zinc-air cells, a pull-tab covers one or more small openings that allow air to reach the air-cathode assembly. The pull-tab may be designed to allow air to diffuse slowly into the cell. With the pull-tab sealing the cell, the cell is oxygen deprived and may not support the same current as an unsealed cell.
The chemical reaction associated with an oxygen-enriched zinc-air cell is as follows:
When a cell is completely deprived of oxygen, the cell becomes a zinc-hydroxide cell, wherein the cathode material is hydroxide taken from the electrolyte. The chemical reaction associated with the zinc-hydroxide cell is as follows:
A pull-tab that is impermeable to oxygen may be used to seal the air openings. Instead of an oxygen impermeable pull-tab, or in addition to such a pull-tab, the cell (battery) may be sealed in a nitrogen-filled, oxygen impermeable bag. The relative humidity of the nitrogen gas within the bag may be, for example, between 40 and 60 percent so as not to dry out the cell. When the sealed bag is opened or the pull-tab is removed, oxygen diffuses into the cell, the cell reverts to a zinc-air cell, and the voltage may increase, for example, from about 0.39 volts to more than 4 volts.
Hearing aids may typically be designed to operate in a range varying from approximately 1.5 volts down to approximately 1.1 volts. Batteries are replaced when the voltage in the battery falls below 1.1 volts. U.S. Pat. No. 5,712,919, issued to Ruhling discloses a hearing aid powered by a single capacitor or several capacitors connected in parallel and operating at 1.5 volts. When the capacitor is discharged to about 1.1 volts and no longer useful, only 46.2% of its energy has actually been used.
Assuming a constant current load, the operating life of the capacitor- powered hearing aid is given by:
xcex94t=Cxc2x7xcex94v/Ixe2x80x83xe2x80x83(1)
where C is the capacitance, xcex94v is allowable voltage drop (e.g. 1.5Vxe2x88x921.1V=0.4V), and I is the load current. Equation (1) may be rearranged to calculate the capacitance for a desired operating time as follows:
C=Ixc2x7xcex94t/xcex94vxe2x80x83xe2x80x83(2)
for example, xcex94v=0.4 volts, xcex94t=1 day (86400 seconds), and I=500 xcexcA (micro-amperes), a capacitance of 108 F (farads) is realized. Double-layer capacitors which have large capacitances relative to their sizes are commercially available. For example, Panasonic, Part Number EEC-A0EL106, is a 10 F, 2.5 V capacitor. By using eleven of these capacitors a total capacitance of 110 F may be obtained. The total physical volume of eleven such capacitors is 98.0 cm3. It will be appreciated that a typical in-the-canal (ITC) hearing aid may occupy a volume of only about 1 cm3. Thus, the configuration described may not be feasible for use in an ITC hearing aid.
As previously stated, the difficulty with the disposable hearing aid is that its permanent battery may discharge during the shelf-life period. To ensure that the hearing aid lasts for its target life of 30 days, for example, a switch may be included in the device to keep the battery from discharging. Two types of switches may be used: an xe2x80x9con-offxe2x80x9d switch or an xe2x80x9con-onlyxe2x80x9d switch. An xe2x80x9con-onlyxe2x80x9d switch may be used to activate the device once. Once put into service the device remains xe2x80x9con.xe2x80x9d An xe2x80x9con-offxe2x80x9d switch, in addition to activating the device once, may allow the hearing aid to be turned off during non-use periods, for example at sleep time.
The present invention provides a metal-air cell for powering electronic components in a hearing aid device. The metal-air cell has a flex circuit inside a housing containing an anode mixture. The flex circuit comprises a flexible substrate having a cathode electrode area at one end of the substrate, an anode electrode area at another end of the substrate and an electronic components area in between. The flexible substrate is disposed within the housing. The cathode electrode area is near a top surface of the housing, and the anode electrode area is near a bottom surface of the housing and in contact with the anode mixture. The cathode electrode area has air flow means for permitting air into the housing. An isolation means between the cathode electrode area and the anode mixture is provided for preventing contact between the cathode electrode area and the anode mixture.
In another embodiment, this invention provides an automatic switch for controlling power in a hearing aid having a load and a battery source comprising
(1) a voltage comparator connected to the battery source and having a reference voltage level of comparing a voltage level of the battery source to the reference voltage level to generate a control signal, and (2) a switch responsive to the control signal to selectively connect the battery source to the load. The switch connects the battery source to the load when the battery voltage level exceeds the reference voltage level by a predetermined voltage and the switch disconnects the battery source from the load when the battery voltage level is below the reference voltage level.
In yet another embodiment, this invention provides a source of operating potential supplying a predetermined voltage level to a load in a hearing aid comprising at least one storage capacitor having a stored voltage level higher than the predetermined voltage level, and
a DC/DC converter connected between the load and the storage capacitor for converting the stored voltage level to the predetermined voltage level.
In still another embodiment, this invention provides an xe2x80x9con-offxe2x80x9d switch. In an in-the-canal hearing aid device having a metal air cathode isolated from an internal circuit and microphone and a cathode eyelet press-fitted into the metal air cathode to form an opening for the microphone. The xe2x80x9con-offxe2x80x9d switch comprises (1) a first spring electrically connected to the internal circuit at a first end and disposed adjacent to the cathode eyelet at a second end, (2) a second spring electrically connected to the cathode eyelet at a third end and urging away from the second end at a fourth end toward, and (3) actuating means positioned at the second spring for selectively moving the second end. The metal air cathode is electrically connected to the internal circuit when the actuating means moves the fourth end toward the second end of the metal air cathode and is electrically isolated from the internal circuit when the actuating means moves the fourth end away from the second end.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.